Amine-terminated reactive liquid polymers prepared from the derivitization of functional-terminated reactive liquid polymers, and their use in modifying epoxy resins are known. For example, U.S. Pat. No. 4,133,957 discloses amine-terminated liquid polymers produced by reacting an aliphatic, alicyclic, heterocyclic or aromatic amine containing at least two secondary or mixed primary/secondary amine groups, but no more than one primary amine group per molecule, with a carboxyl-terminated, ester-terminated or acid chloride-terminated liquid polymer. Such polymers, particularly amine-terminated liquid nitrile rubbers, have found utility as epoxy resin modifiers for the purposes of toughening or flexibilizing. They have been employed with amine cure agents of choice in many different epoxy systems for a wide variety of applications ranging from adhesives to coatings to encapsulants.
However, some problems have been encountered by epoxy resin formulators when formulating with these conventional amine-terminated polymers in that they have high viscosities and relatively slow secondary amine reactivity. High viscosity polymeric modifiers are undesirable to work with because they are difficult to pour, mix and process in general. The presence of the secondary amine makes the polymer less reactive toward the epoxy.
In order to obtain a workable viscosity, such polymers would have to be made by using a large excess of a diprimary amine as disclosed in U.S. Pat. No. 3,823,107. Typically, the viscosity of the AT-RLP and the amount of excess amine which is present in the AT-RLP are inversely related: viscosity decreases as residual amine content increases. It is believed that the excess amine acts as a diluent to lower viscosity. However, a relatively large excess residual amine limits the utility of the AT-RLP to strictly that of a curing agent, because it reacts with the epoxy at a stoichiometric cure such that a co-amine modifier cannot be used. Consequently, the epoxy system cannot be modified beyond the physical property characteristics imparted by the curing agent. Therefore, an epoxy resin formulator, desirous of achieving certain physical properties, is limited by the choice of such a curing agent containing a large excess amine.
It has now surprisingly been found that a low viscosity amine-terminated reactive liquid polymer having only primary amine reactive groups and having low excess amine can be achieved by reacting certain diprimary amines, defined hereinbelow, with functional-terminated reactive liquid polymers.
Consequently, the AT-RLPs of this invention solve the above-mentioned problem traditionally encountered by users of previously known polymers of higher viscosities. The viscosity of these new modifiers can be less than half that of conventional polymers rendering them easier to handle and process. Additionally, the rate of reactivity of the primary amine terminal groups with the epoxy resin is enhanced over the previously known polymers containing secondary amine groups. Furthermore, the use of these AT-RLPs with low residual amine permits the use of a co-amine or curing agent to bring the system into stoichiometric balance. Consequently, a formulator can use these AT-RLPs in the desired quantity to achieve desired properties, such as peel strength of an adhesive, and additionally be selective with respect to a curing agent.